1. Field of the Invention
The present invention relates a throttle system abnormality determination apparatus for determining existence or nonexistence of an abnormality in a throttle control system, which controls a throttle valve of an internal combustion engine, or in parts related to the throttle control system.
2. Description of Related Art
A certain internal combustion engine mounted to a vehicle has an electronic throttle control system for electronically controlling a throttle angle (an opening degree of a throttle valve) TH. The system includes a throttle motor for driving the throttle valve. The system calculates a target throttle angle based on an amount of manipulation of an accelerator and others, and senses an actual throttle angle by a throttle position sensor. Thus, the system controls the throttle motor to make the actual throttle angle follow the target throttle angle.
An abnormality determination apparatus for the above throttle control system disclosed in Japanese Patent Gazette No. H07-74623 (page 1, etc.) (Patent Document 1) calculates an integrated value DEV of a deviation between the actual throttle angle and the target throttle angle and compares the integrated value DEV with a predetermined abnormality determination value. Thus, the abnormality determination apparatus determines existence or nonexistence of an abnormality in the throttle control system. The abnormality determination apparatus of Patent Document 1 changes the abnormality determination value in accordance with temperature of the throttle motor when the apparatus determines the existence or nonexistence of the abnormality. Thus, the abnormality determination apparatus avoids erroneous determination due to a delay in response, which is caused when the temperature of the throttle motor is low.
Generally, if it is determined that the abnormality exists in the throttle control system, the abnormality determination apparatus of the throttle control system performs failsafe processing for turning off energization of the throttle motor and for compulsorily holding the throttle angle at a predetermined throttle angle by a spring and the like. For instance, the predetermined throttle angle is an angle at which an air intake quantity necessary for the vehicle to travel to a safe place can be ensured.
Generally, the target throttle angle becomes smaller in a low-speed traveling period (in which a vehicle travels at a low speed) than in a high-speed traveling period (in which the vehicle travels at a high speed). Therefore, even if an amount of abnormal operation of the actual throttle angle TH due to the abnormality in the throttle control system is the same, a ratio of the deviation of the actual throttle angle with respect to the target throttle angle becomes greater in the low-speed traveling period than in the high-speed traveling period. Therefore, a ratio of a change in an operating state of an internal combustion engine (for instance, a ratio of a change in a vehicle speed Vs) caused by the abnormal operation of the actual throttle angle is apt to increase in the low-speed traveling period.
In the abnormality determination performed by the throttle control system of Patent Document 1, the abnormality determination value becomes constant when the temperature of the throttle motor becomes constant after warming up of the engine finishes. Therefore, the abnormality determination value shown by a double-dashed line FLD′ in FIG. 4 used in the high-speed traveling period is also used in the low-speed traveling period. In FIG. 4, a broken line THtgt indicates the target throttle angle. In FIG. 4, a solid line THact′ indicates the actual throttle angle of the throttle control system of Patent Document 1, and a solid line DEV′ is the integrated value of the deviation between the target throttle angle THtgt and the actual throttle angle THact′. If an abnormality in which the actual throttle angle THact′ increases with respect to the target throttle angle THtgt occurs in the throttle control system of Patent Document 1, it will be determined that the abnormality exists at a time point t1 when the integrated value DEV′ exceeds the abnormality determination value FLD′, which is the same as the abnormality determination value FLD′ used in the high-speed traveling period, even in the low-speed traveling period. Then, the failsafe processing will be started. A flag FLAG′ indicating the existence of the abnormality is set at 1 at the time point t1 as shown by a solid line f′ in FIG. 4. However, as explained above, the abnormality in the throttle control system occurring in the low-speed period has a significant influence on the operating state of the engine. Therefore, the rate of the increase in the vehicle speed Vs due to the abnormal operation of the actual throttle angle THact′, which is performed during a period from a time point when the abnormality is generated to the time point t1 when it is determined that the abnormality exists, is increased as shown by a solid line v′ in FIG. 4. As a result, there is a possibility that a vehicle driver feels deterioration of drivability such as acceleration shock.